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You're speeding though space 60,000 miles an hour.
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00:00:12,784 --> 00:00:14,419
Precisely a year from now,
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you will have travelled 584 million miles -
to end up exactly where you started.
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00:00:21,760 --> 00:00:28,767
In 365 clays, all of us on planet Earth make
this epic journey once around the sun.
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But because of the orbit takes around its star,
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the way our planet spins, and the way it's tilted over,
our journey brings with it extreme weather.
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Did you see that'? The whole house blew apart.
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It produces bizarre phenomena.
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And drives earth's diverse climates.
10
00:01:04,269 --> 00:01:08,106
The connection between
the Earth and the Sun defines our planet.
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00:01:12,411 --> 00:01:14,379
It's changed the course of human history.
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00:01:21,853 --> 00:01:25,657
It's responsible for the story of life itself.
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00:01:44,142 --> 00:01:47,646
Our lives are divided into 24-hour periods.
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For some of that time it's dark,
and for the rest of the time it's light.
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Day and night are easily explained by the fact
that the Earth spins on its axis as it orbits the sun.
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But what happens in the Arctic
shows that things aren't quite so simple.
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Because here in the summer time
there is only day.
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The sun never sets at all.
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And it never sets because of something
that happened to our planet...
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...four and a half billion years ago.
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When our solar system began,
it was a crowded and dangerous place.
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Earth was just one of thousands of
new planets orbiting the young Sun.
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And it was on a collision course with
another, smaller planet called Thea.
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Thea was totally destroyed.
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Debris from the crash
eventually formed our Moon.
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Earth survived,
but the crash had changed it forever.
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Thea knocked the Earth over.
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Our planet spins around its axis at
an angle of 23 degrees away from the upright.
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This 23-degree tilt means that
as Earth orbits the Sun,
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different parts of it get
more or less exposure to the Sun.
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In June and July,
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the tilt means that the northern hemisphere
is facing towards the Sun.
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It's summertime.
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But in the south, the opposite is true.
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The southern hemisphere
is getting far less sunlight.
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It's the southern winter.
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Comem December and January,
the Earth is on the opposite side of the Sun,
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so the tilt means it's winter in the north
and summer in the south.
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00:04:29,841 --> 00:04:34,513
At the poles, the sun never sets in summer
and never rises in winter.
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But it's not just at the extreme ends of the planet
that the tilt is significant.
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For the rest of us in between,
the length of our days changes through the year,
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because of the Earth's tilt.
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Tilt brings us spring, summer, winter an fall.
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Without it, there would be no seasons.
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As the seasons change up and down
the surface of the Earth,
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they bring on dramatic and spectacular events.
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From the intense storms of Tornado Alley.
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We're watching some very strong rotation here.
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This purple area here is winds
about 150 kilometres towards the radar.
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To the dramatic spring
break-up of ice on frozen rivers.
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L've been watching it for 62 years.
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You're going to be faced with
high water and river flow ice.
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You've got nothing at all except a flood.
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From the mass migration of Arctic animals.
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00:05:49,487 --> 00:05:51,990
To the Indian subcontinent's monsoon.
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00:05:53,091 --> 00:05:56,995
This is all the water that's been lifted up
off the ocean in the past few clays,
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because of the heat and been carried inland
and is now falling on top of me.
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00:06:05,036 --> 00:06:09,975
All these phenomena are
a direct result of the Earth's 23-degree tilt.
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00:06:22,554 --> 00:06:29,728
It's March 20th, and crowds of people are gathering
in the ancient Mayan city of Chichen ltza in Mexico.
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00:06:31,463 --> 00:06:35,734
They've come to witness an event
that's inextricably linked to the tilt of the Earth.
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And event so significant,
this, the great pyramid of Kukulkan,
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was built 1200 years ago to capture it.
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It doesn't take much to work out that
the temple has something to do with the year.
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There are four sides which represent
the four seasons.
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Each has 91 steps.
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These, combined with the top platform,
make a total of 365 -
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one for every day of the year.
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But the temple is more than
an ancient stone calendar.
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It also records the solstices,
the shortest and longest days of the year,
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the extremes of the Earth's tilt relative to the sun.
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And it records the spring and fall equinoxes,
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when for a moment
there is no tilt relative to the Sun.
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Today is the spring equinox,
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a point in the year when day and night
are of equal duration.
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The crowds at Chichen ltza are waiting for
a magical transformation...
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...which only happens here
on the day of the equinox.
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As the afternoon sun aligns with the pyramid,
it casts a shadow along the edge of this staircase.
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As it develops, the shadow provides the carved
snake's head at the bottom of the steps with a body.
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This is Kukulkan, a heavenly serpent,
divine messenger of the Mayan sun god.
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The ancient Mayans realised that the equinox
was a significant point in the year,
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and understood enough about the seasons
to capture this moment in stone.
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But they didn't know why it was significant.
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They didn't know that this is the point in its orbit
at which the Earth's tilt is irrelevant.
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Because it's axis is momentarily
at right angles to the Sun,
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neither pole is tipped towards it,
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00:09:17,929 --> 00:09:21,566
and so everywhere on Earth receives
the same amount of night and day.
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00:09:23,701 --> 00:09:28,440
From this clay forward,
the Earth's 23-degree tilt will reassert itself.
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00:09:29,808 --> 00:09:33,511
As the northern hemisphere tilts
more and more towards the Sun,
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the days become longer and warmer.
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00:09:36,181 --> 00:09:41,586
Unsurprisingly, this turning point has been
traditionally marked as a time for renewal.
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00:09:48,827 --> 00:09:51,696
From the pagan dawn ceremony at Stonehenge -
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to the burning of the wicker man
in Northern Ireland.
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And that Catholic fiesta de las Fallas
in Valencia, Spain.
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00:10:18,656 --> 00:10:21,926
As the northern hemisphere tilts
further towards the Sun,
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the extra heat and light
have a dramatic effect on animals and plants.
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00:10:30,135 --> 00:10:35,306
Longer spring days trigger breeding cycles by
stimulating reproductive hormones,
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00:10:36,141 --> 00:10:39,844
and stronger sun also
stimulates trees and plants to grow,
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and the green creeps north
as the global tilt increases.
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Winter has covered the far north in snow,
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and now it is melting back towards the Arctic circle.
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Spring is about to arrive in dramatic fashion.
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This is the Hay river, which flows for
hundreds of miles through northern Canada.
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At its mouth, bordering Great Slave Lake,
lies the town of Hay River.
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In winter, both lake and river
are covered by a thick layer of ice.
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00:11:23,821 --> 00:11:27,058
In spring, as the Earth's tilt
brings the Sun northward,
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the frozen river starts to melt and break up.
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It's an annual event that former Hay River mayor,
Red McBryan, has witnessed for most of his life.
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L've been watching it for 62 years.
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You're going to be faced with
high water and river flow ice.
110
00:11:47,545 --> 00:11:49,914
You've got nothing at all except a flood.
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00:11:51,149 --> 00:11:55,920
Flooding happens because the river flows
from the warmer south to the colder north.
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Great Slave Lake is still frozen solid when blocks of
melting ice arrive from upstream to the south.
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This forms ice dams which can force
the river over its banks, flooding the town.
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The trouble is, though,
no one knows if the flood will happen or not.
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Which is why Jennifer Nafziger...
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...has been brought in to take
some of the uncertainty out of the Hay River spring.
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We're trying to help the town of Hay River
understand how much flooding...
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...that they might have in town
and when that flooding might occur.
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Jennifer and her team
are using time lapse photography...
120
00:12:41,532 --> 00:12:43,668
...to monitor the break-up of the river ice.
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In particular, they're interested
in this waterfall 30 miles upriver from the town.
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When ice starts to flow
from the river over these falls,
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that's the indication for the town
that break-up is going to happen,
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and that's when the town should be
starting to do their flood watch operation.
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00:13:19,971 --> 00:13:24,075
This year when the falls were breached
the town escaped unscathed.
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00:13:25,543 --> 00:13:28,046
Jennifer hopes that the data she and her team...
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...have gathered will enable
future accurate prediction of flooding.
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But it'll take more data and more time.
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00:13:39,657 --> 00:13:45,663
After all, she's up against a whole chain of events
that started four and a half billion years ago,
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when the Earth's 23-degree tilt was born.
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00:13:50,134 --> 00:13:52,870
Next year it may be all hell to pay.
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It may be two feet thick, higher water,
133
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and she'll just go through here and -
and play hell with us.
134
00:14:05,850 --> 00:14:11,689
As spring progresses towards summer,
the Earth's tilt begins to thaw the Arctic itself.
135
00:14:15,193 --> 00:14:18,062
Above the Hay River, caribou are on the move,
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00:14:18,363 --> 00:14:22,333
following the sun north towards the tundra,
newly free of snow.
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00:14:24,602 --> 00:14:28,039
Pregnant females arrive first and deliver their calves.
138
00:14:30,141 --> 00:14:32,910
Within a couple of weeks,
the rest of the herd gathers.
139
00:14:33,378 --> 00:14:39,117
Caribou herds across the north will feed and
fatten up during the endless daylight of Arctic summer.
140
00:14:47,759 --> 00:14:51,796
At the other end of the planet,
the opposite seasonal change is under way.
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00:14:52,463 --> 00:14:56,934
As the Earth's tilt draws the southern hemisphere
farther and farther away from the sun,
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temperatures plummet and the days shorten.
143
00:15:04,609 --> 00:15:08,246
In Antarctica, another epic migration is under way.
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00:15:08,546 --> 00:15:12,250
Humpback whales have been feeding here
during Antarctic summer.
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00:15:12,750 --> 00:15:15,887
Now, with the approach of winter,
they head north...
146
00:15:16,154 --> 00:15:18,856
...to warmer breeding grounds
off the coast of Australia.
147
00:15:21,526 --> 00:15:24,996
But as some species abandon
the Antarctic's punishing winter,
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others use it to their advantage.
149
00:15:28,566 --> 00:15:32,003
The days are much shorter and colder now
in the southern hemisphere.
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00:15:33,004 --> 00:15:35,807
It's a perfect time for emperor penguins to mate,
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because the growing sea ice is finally strong enough
to support their vast breeding colonies.
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And it provides a place of
relative safety for the young.
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All over the planet,
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the natural world reacts to the Earth's
changing relationship to the Sun...
155
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...as its 23-degree tilt
becomes more or less significant.
156
00:16:10,141 --> 00:16:12,877
As spring turns to summer
in the northern hemisphere,
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the sun's rays warm the earth producing thermals,
158
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the updrafts of warm air
beloved of para-glider pilots.
159
00:16:22,820 --> 00:16:25,756
We're finally getting above launch height.
160
00:16:26,023 --> 00:16:27,225
Better check our altitude.
161
00:16:29,961 --> 00:16:31,796
We're 2300 metres.
162
00:16:32,663 --> 00:16:34,098
Let's see how high we can take this one.
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00:16:35,199 --> 00:16:39,437
Paragliding champion Hunter Ramanek
is no ordinary birdman.
164
00:16:39,971 --> 00:16:44,642
He's also a meteorologist,
so understands the atmosphere better than most.
165
00:16:49,981 --> 00:16:51,382
Hunter's great skill...
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...is to ride the complex currents of
air stirred up by the Rocky Mountains beneath him.
167
00:16:58,055 --> 00:16:59,190
At this time of the year,
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solar-powered thermals are another force
he can exploit to keep airborne.
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00:17:05,229 --> 00:17:09,166
Thermals exist because the ground
has been heated by the sun,
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and the ground in turn heats the air that overlies it.
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00:17:14,805 --> 00:17:18,409
A thermal is a warm rising plume of
air that is invisible,
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but is ascending upwards at
sometimes several hundred feet per minute.
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00:17:26,484 --> 00:17:32,456
Hunter is exploiting the fundamental principle of
atmospheric physics - that hot air rises.
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00:17:34,759 --> 00:17:37,895
When air warms and expands,
it becomes less dense.
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00:17:38,296 --> 00:17:41,732
As it floats upwards,
it forms a rising thermal column.
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It's a very powerful feeling to be spiralling skyward
on something that's just created by the sun.
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But thermals do a lot more than
just keep paraglider pilots happy.
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As they rise,
thermals eventually mix with the cooler air above.
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00:18:04,755 --> 00:18:08,259
Often the rising warm air
carries water vapour with it.
180
00:18:13,030 --> 00:18:15,099
When it hits the colder air aloft,
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00:18:15,399 --> 00:18:20,905
the water vapour is transformed into
a mass of tiny droplets and a cloud is formed.
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00:18:23,808 --> 00:18:28,980
This basic process lies at the heart of
every significant weather event on the planet.
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It's the tilt of the Earth
that adds intensity to this thermal activity,
184
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especially as it changes the seasons.
185
00:18:47,398 --> 00:18:50,334
In springtime up and down the American Midwest,
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there's a regional phenomenon that demonstrates
the thermal process in a dramatic way.
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Spring brings warm, wet air masses from the south,
188
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creating the perfect conditions for
one of the planet's most severe weather events.
189
00:19:09,587 --> 00:19:13,791
It's why this part of the Midwest
is called Tornado Alley.
190
00:19:28,739 --> 00:19:33,044
2011 was one of the worst tornado seasons
in living memory,
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with destruction on a scale rarely seen.
192
00:19:37,748 --> 00:19:38,582
Did you see that?
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The whole house blew apart.
194
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Oh my god.
195
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Oh my god.
196
00:19:48,392 --> 00:19:56,233
In Joplin, Missouri, a tornado cut a six mile-wide path
of damage, killing more than 160 people.
197
00:19:57,968 --> 00:20:00,471
But what's really terrifying about tornadoes...
198
00:20:00,805 --> 00:20:04,542
...is that though the science behind
their power and operation is well known,
199
00:20:05,476 --> 00:20:09,847
accurately predicting them
remains tantalisingly out of reach.
200
00:20:18,956 --> 00:20:22,059
Josh Wurman is an atmospheric scientist.
201
00:20:22,626 --> 00:20:26,597
He and his team from the Centre for
Severe Weather Research in Colorado...
202
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...are investigating how
thunderstorms produce tornadoes.
203
00:20:33,337 --> 00:20:37,141
Tornadoes depend on the transition of seasons.
204
00:20:37,508 --> 00:20:40,678
We need summery conditions at the surface,
205
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a lot of warmth and humidity
that gives us energy for thunderstorms.
206
00:20:44,081 --> 00:20:47,518
But then we still need a wintertime jet stream,
207
00:20:47,752 --> 00:20:49,487
we still need the strong jet stream...
208
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...which is caused by the contrast of
tropical temperatures and the still cold Arctic.
209
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So we need that combination of conditions,
210
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which only really exists
during the transition season of spring.
211
00:21:07,104 --> 00:21:08,439
The missing piece of the puzzle that...
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00:21:08,706 --> 00:21:14,378
...we're still actively researching is
exactly how does storms form the tornadoes?
213
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About three quarters of them don't.
214
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But a quarter of them do,
215
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and we have a very hard time predicting exactly
which ones of those are going to make tornadoes,
216
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when they're going to make tornadoes,
217
00:21:25,856 --> 00:21:28,993
and which ones are going to make
the strongest and most violent tornadoes.
218
00:21:31,962 --> 00:21:33,898
To get answers to these questions,
219
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Josh and his team need to record data from
as many tornadoes as they can during the season.
220
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To hunt down developing storms,
Josh uses a Doppler radar scanner.
221
00:21:45,976 --> 00:21:48,813
It can show him what's going on
inside a thunderstorm,
222
00:21:49,079 --> 00:21:54,118
giving him critical information
about the conditions needed to trigger a tornado.
223
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The Doppler radar is able to send a beam of
microwaves out through a tornado...
224
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...and make maps of where it's raining,
how hard it's raining,
225
00:22:04,695 --> 00:22:06,564
whether the winds are going strong,
226
00:22:06,831 --> 00:22:08,933
whether they're weaker
and which direction the winds are going,
227
00:22:09,233 --> 00:22:11,135
and watch how a tornado is evolving.
228
00:22:11,869 --> 00:22:14,772
As the Earth's tilt gradually points
the northern hemisphere...
229
00:22:15,039 --> 00:22:20,511
...further towards the Sun and spring progresses,
tornado activity moves north.
230
00:22:21,312 --> 00:22:27,518
But although Josh knows roughly where to look,
finding individual storms is always difficult.
231
00:22:28,652 --> 00:22:33,991
Then, after the team has driven over
a thousand miles and have crossed six state lines,
232
00:22:34,258 --> 00:22:36,560
Josh's radar detects a storm.
233
00:22:37,161 --> 00:22:40,364
It's beginning to show
the tell-tale movement of clouds.
234
00:22:42,833 --> 00:22:47,304
The team has to move fast:
tornadoes form and vanish very quickly.
235
00:22:48,339 --> 00:22:50,307
We're watching some very strong rotation here,
236
00:22:50,541 --> 00:22:54,411
this purple area here is winds
about 150 kilometres towards the radar.
237
00:22:54,678 --> 00:22:57,381
We're also watching this kind of swirl down here...
238
00:22:57,615 --> 00:23:01,418
...which is associated with a rotation
that's coming up the road towards us.
239
00:23:06,090 --> 00:23:12,529
High above the Midwest, this thunderstorm has hit
the jet stream, which has made it start to spin.
240
00:23:13,564 --> 00:23:17,768
The storm becomes a giant rotating air mass,
a super-cell.
241
00:23:25,009 --> 00:23:27,278
Well, right now we're intercepting a super-cell...
242
00:23:27,444 --> 00:23:30,514
...that is probably made from
small, short-lived tornadoes.
243
00:23:32,049 --> 00:23:34,351
Going on ahead, this big dark area is the core.
244
00:23:34,685 --> 00:23:38,923
So we're basically going to penetrate
through the core and see what's interesting.
245
00:23:39,690 --> 00:23:42,960
Yeah, right now we're kind of right
in the centre of that coiled part of it.
246
00:23:48,265 --> 00:23:50,868
When the spinning column of air
touches the ground,
247
00:23:51,235 --> 00:23:52,603
a tornado is born.
248
00:23:54,505 --> 00:24:01,445
At the tornado's rotating core is an area of intense
low pressure which draws high pressure air towards it.
249
00:24:02,046 --> 00:24:04,415
Normally this is an invisible process.
250
00:24:04,682 --> 00:24:07,918
But a tornado sucks in dust and debris.
251
00:24:08,819 --> 00:24:11,488
There we go, that's what it's about.
252
00:24:23,300 --> 00:24:25,669
This is what all the effort has been for.
253
00:24:25,869 --> 00:24:28,505
A huge tornado less than a mile ahead.
254
00:24:32,776 --> 00:24:35,946
It's one of the most dramatic
consequences of the Earth's tilt...
255
00:24:36,313 --> 00:24:41,018
...and what can happen in the northern hemisphere
when spring collides with winter.
256
00:24:41,885 --> 00:24:46,724
In this case, the Earth's tilt had given birth
to an F3 rated tornado,
257
00:24:46,957 --> 00:24:50,394
with winds travelling at up to 200 miles per hour.
258
00:25:01,538 --> 00:25:07,344
But less than half an hour
after it first formed, the tornado dies.
259
00:25:08,379 --> 00:25:11,181
It was just one of more than 1200 tornadoes...
260
00:25:11,515 --> 00:25:14,151
...that have battered this region
since the beginning of spring.
261
00:25:17,221 --> 00:25:19,490
This year more than any year in recent memory,
262
00:25:19,690 --> 00:25:22,459
we've seen how bad tornadoes can be.
263
00:25:22,726 --> 00:25:26,764
We've seen tornadoes go through cities,
we've had over 500 people killed so far.
264
00:25:27,131 --> 00:25:33,037
If we can increase the accuracy of warnings,
then people may have time to get to better shelter.
265
00:25:39,343 --> 00:25:42,513
Tornadoes are not the only
consequence of our planet's tilt...
266
00:25:42,713 --> 00:25:45,983
...driving the onset of spring
in the northern hemisphere.
267
00:25:47,317 --> 00:25:52,089
The unstable atmosphere caused by
warming air masses as they invade the north,
268
00:25:52,389 --> 00:25:55,359
triggers other big weather events
around the hemisphere.
269
00:25:57,027 --> 00:26:02,199
Storms can cause heavy and sudden downpours,
which can result in flash floods.
270
00:26:02,833 --> 00:26:07,204
These occur when heavy rain hits ground
already saturated with water.
271
00:26:13,944 --> 00:26:18,982
Seasonal thunderstorms can also give birth
to a phenomenon called a haboob,
272
00:26:19,349 --> 00:26:21,785
like this one in Phoenix, Arizona.
273
00:26:22,853 --> 00:26:29,359
These massive dust storms occur in arid regions
when the leading edge of a thunderstorm collapses,
274
00:26:29,660 --> 00:26:32,329
generating a superfast downdraught.
275
00:26:33,030 --> 00:26:37,034
Falling winds kick up a wall of
dust and sand in front of it.
276
00:26:42,005 --> 00:26:43,307
As summer progresses,
277
00:26:43,574 --> 00:26:48,112
the planet's tilt pushes the northern hemisphere
even further towards the sun.
278
00:26:50,247 --> 00:26:53,784
This brings even more solar energy
to the warming land...
279
00:26:53,984 --> 00:26:58,155
...and generates the biggest
single weather event on the planet.
280
00:27:06,997 --> 00:27:08,999
Oceanographer Helen Czerski...
281
00:27:09,233 --> 00:27:14,771
...has come to Kerala on lndia's southern coast
to witness the start of the monsoon.
282
00:27:16,173 --> 00:27:20,244
We're on the edge, and the land -
the big Indian land mass behind me,
283
00:27:20,544 --> 00:27:21,645
the ocean's to the south,
284
00:27:21,879 --> 00:27:24,314
and this is where the monsoon is set up.
285
00:27:27,618 --> 00:27:30,954
Whats driving the monsoon
is the difference between the land and the sea.
286
00:27:31,255 --> 00:27:34,024
So the sun is beating down on this area of land,
287
00:27:34,224 --> 00:27:36,894
especially at this time of year
when the sun's almost directly overhead.
288
00:27:37,361 --> 00:27:42,866
And the land heats up very, very quickly,
whereas the ocean heats up much more slowly.
289
00:27:45,335 --> 00:27:49,273
If you look at the sand here,
the surface has been heated by the sun.
290
00:27:49,473 --> 00:27:50,107
It's hot.
291
00:27:50,440 --> 00:27:52,576
And if you dig down,
you don't have to dig down very far,
292
00:27:53,210 --> 00:27:58,248
even a little way down that sand's really cool,
whereas this layer here is really, really hot.
293
00:27:58,448 --> 00:28:00,717
And you can see that
only a thin layer is being heated,
294
00:28:01,018 --> 00:28:04,888
so all of that sun's energy that's being absorbed
is going into this really thin layer...
295
00:28:05,222 --> 00:28:06,423
...so it heats up very, very quickly.
296
00:28:10,861 --> 00:28:15,332
So the sun heats the land dramatically,
but not the ocean.
297
00:28:16,700 --> 00:28:20,003
Here, the Indian Ocean is
much cooler than the land.
298
00:28:20,437 --> 00:28:24,975
The reason is the ocean requires
much more of the sun's energy to heat it up.
299
00:28:28,445 --> 00:28:31,548
The ocean is also relatively cool
because to heat the surface,
300
00:28:31,848 --> 00:28:34,051
you have to heat much more than just a thin layer.
301
00:28:34,418 --> 00:28:37,688
What happens is that winds that blow
across the surface of the ocean...
302
00:28:37,955 --> 00:28:40,524
...generate turbulence which mixes that top layer.
303
00:28:40,891 --> 00:28:44,561
So as soon as some water's been heated
at the top it gets mixed down below.
304
00:28:48,365 --> 00:28:54,671
As we enter summer, the land heats up fast,
while the ocean lags further and further behind.
305
00:28:56,173 --> 00:29:00,544
Now the air above the land becomes
much hotter than the air above the ocean.
306
00:29:01,044 --> 00:29:04,481
As the warmer air mass rises
above the Indian subcontinent,
307
00:29:04,815 --> 00:29:07,084
it creates a kind of vacuum behind it.
308
00:29:07,784 --> 00:29:11,788
This draws in the cooler, denser air
from the ocean to replace it.
309
00:29:13,357 --> 00:29:18,228
Because of lndia's particular geography,
the process happens on a huge scale.
310
00:29:18,695 --> 00:29:24,901
It's a triangular peninsular with both wide
hot plains and, crucially, a very long coastline.
311
00:29:27,704 --> 00:29:33,210
This combination sets up a powerful and sustained
movement of cooler ocean air towards land,
312
00:29:33,543 --> 00:29:35,145
called the monsoon wind.
313
00:29:37,948 --> 00:29:41,752
But what we think of with monsoons
is not wind, but rain.
314
00:29:42,786 --> 00:29:44,254
There's an enormous cloud overhead,
315
00:29:44,554 --> 00:29:48,925
an enormous black cloud which has built
15 kilometres up into the troposphere,
316
00:29:49,126 --> 00:29:51,862
and the first drops of rain are
just starting to come down.
317
00:29:54,998 --> 00:29:57,067
As the sun beats down on the ocean,
318
00:29:57,301 --> 00:30:00,671
some of the water evaporates
and is carried up into the atmosphere,
319
00:30:00,937 --> 00:30:02,939
where it forms vast rainclouds.
320
00:30:08,679 --> 00:30:12,916
To capture their formation,
Helen has set up a series of time lapse cameras.
321
00:30:22,659 --> 00:30:26,596
The monsoon winds drive
the moisture-laden clouds across the land,
322
00:30:26,897 --> 00:30:29,466
bringing with them a torrential downpour.
323
00:30:31,601 --> 00:30:36,006
Eighty per cent of all lndia's rains
arrive in this seasonal deluge.
324
00:30:37,274 --> 00:30:38,809
This is the monsoon.
325
00:30:39,209 --> 00:30:43,013
This is all the water that's been lifted up
off the ocean in the past few clays...
326
00:30:43,246 --> 00:30:47,250
...because of the heat and has been carried inland
and is now falling on top of me.
327
00:30:49,353 --> 00:30:51,388
The raindrops here are really, really large,
328
00:30:51,621 --> 00:30:54,324
and you can see how they leave pockmarks
in the sand because they're so big.
329
00:30:54,558 --> 00:30:56,760
And the reason for that is
in warm conditions like this...
330
00:30:57,160 --> 00:31:00,063
...up in the clouds the droplets are
joining together to make bigger drops.
331
00:31:00,397 --> 00:31:02,366
The smaller drops actually stay up in the clouds,
332
00:31:02,699 --> 00:31:03,834
they're being lifted out of the way...
333
00:31:04,034 --> 00:31:06,670
...and all that falls are these massive drops
that are covering me now.
334
00:31:10,440 --> 00:31:15,579
The monsoon is an epic weather event of huge
importance to the whole subcontinent...
335
00:31:15,812 --> 00:31:18,882
...and the people who live here -
over a billion of them.
336
00:31:32,562 --> 00:31:37,501
Nowhere is this more important than here
in the city of Udaipur in Rajasthan,
337
00:31:37,768 --> 00:31:39,569
part of lndia's north-western corner.
338
00:31:42,072 --> 00:31:44,508
Though it starts in May on the Kerala coast,
339
00:31:44,775 --> 00:31:49,312
the life-giving waters of the monsoon
don't arrive here until mid-July.
340
00:31:49,780 --> 00:31:52,249
Sometimes they don't arrive at all.
341
00:31:58,221 --> 00:32:00,791
Because of the importance of the annual rains,
342
00:32:01,024 --> 00:32:06,229
the 72nd Maharana of Udaipur built Sajanghar,
the monsoon palace.
343
00:32:10,000 --> 00:32:15,405
Sajanghar provides a panoramic view of
the clouds that herald the start of the monsoon,
344
00:32:15,739 --> 00:32:18,175
which is still critical to the region today.
345
00:32:18,675 --> 00:32:23,547
As the 72nd Maharana's descendant,
Prince Lakshyaraj knows well.
346
00:32:26,016 --> 00:32:31,421
The monsoons are very, very important for us,
purely because everybody is dependent on water.
347
00:32:32,589 --> 00:32:34,758
Obviously it makes a huge difference to the farmers.
348
00:32:36,526 --> 00:32:41,164
It can show stability for the region,
for crops, for tourism, for livelihood.
349
00:32:41,465 --> 00:32:42,599
So it's very, very important.
350
00:32:44,100 --> 00:32:47,070
Every drop of monsoon rain is so critical here,
351
00:32:47,337 --> 00:32:50,974
and the prince's ancestors didn't stop
at building a viewing platform,
352
00:32:51,208 --> 00:32:57,514
but went on to build a network of artificial lakes
to capture and store the precious monsoon rain.
353
00:32:59,816 --> 00:33:02,319
Because when the earth tilts back the other way,
354
00:33:02,586 --> 00:33:06,723
there are many months of dry weather
with not a drop of rain.
355
00:33:07,958 --> 00:33:12,963
We're very fortunate for the vision that
the rulers then had of building a lot of lakes...
356
00:33:13,230 --> 00:33:16,766
...in order to be able to store a lot of water
for the people of Udaipur...
357
00:33:16,967 --> 00:33:19,936
...and it's still ensuring
the welfare of the people today.
358
00:33:29,179 --> 00:33:31,648
Monsoon is not unique to India.
359
00:33:31,982 --> 00:33:37,420
There are seasonal monsoon rains
in Africa, Asia, Australia and even North America.
360
00:33:38,555 --> 00:33:42,826
They are all generated by
seasonal variations in surface heating...
361
00:33:43,059 --> 00:33:47,130
...as different parts of the Earth tilt towards
and away from the Sun.
362
00:33:51,768 --> 00:33:55,505
When Thea and the young Earth collided
billions of years ago,
363
00:33:55,839 --> 00:34:00,977
it gave us the seasons, paving the way for
some of the world's most spectacular events.
364
00:34:02,178 --> 00:34:04,247
But that isn't quite the end of the story.
365
00:34:12,656 --> 00:34:14,691
This is the Egyptian Sahara.
366
00:34:15,258 --> 00:34:17,427
It's searingly hot and dry.
367
00:34:18,428 --> 00:34:20,030
But it wasn't always like this.
368
00:34:26,770 --> 00:34:31,575
Hidden in this lifeless landscape is
remarkable evidence of how the planet's tilt...
369
00:34:31,775 --> 00:34:35,478
...has had an impact
on climate, transformed landscapes,
370
00:34:35,679 --> 00:34:38,648
and profoundly altered the course of human history.
371
00:34:49,960 --> 00:34:53,196
Geographer and Palaeo-climatologist Nick Drake...
372
00:34:53,396 --> 00:34:57,767
...is travelling deep into the Sahara close to
the Libyan border to discover...
373
00:34:58,001 --> 00:35:01,871
...how, when and why
the Saharan environment changed.
374
00:35:04,808 --> 00:35:08,712
After three days of travelling,
there's a gradual change in the landscape.
375
00:35:09,145 --> 00:35:12,515
The dunes give way
and hills and ravines begin to appear.
376
00:35:17,821 --> 00:35:23,159
This is the vast limestone plateau
known as the Gilf Kebir, or Great Barrier.
377
00:35:25,228 --> 00:35:30,233
Hidden in this forbidding landscape
are intriguing clues to this desert's past.
378
00:35:40,443 --> 00:35:44,481
This is the Cave of Beasts, discovered in 2002.
379
00:35:44,714 --> 00:35:48,551
It gives us an extraordinary glimpse
into a very different Sahara.
380
00:35:51,955 --> 00:35:56,026
Wow, what a vista of
humans and animals we've got here.
381
00:35:58,228 --> 00:36:01,464
We've got a lovely line of
what appear to be antelopes.
382
00:36:02,932 --> 00:36:04,200
This one look like it's running.
383
00:36:08,204 --> 00:36:09,673
Oh, and here we've got a really nice one.
384
00:36:09,939 --> 00:36:11,541
A nice giraffe there.
385
00:36:13,209 --> 00:36:18,448
And over here what looks like a Barbary sheep,
with these big curly horns.
386
00:36:20,283 --> 00:36:21,284
Here we have a goat.
387
00:36:22,385 --> 00:36:23,486
And these are quite interesting,
388
00:36:23,787 --> 00:36:27,691
because the goats were introduced in the Sahara
sometime between 6,000 and 7,000 years ago.
389
00:36:28,591 --> 00:36:31,561
So it gives us a time frame for this rock art.
390
00:36:37,600 --> 00:36:43,139
These remarkable paintings are
up to 3,000 years older than the pyramids.
391
00:36:43,740 --> 00:36:46,910
But it's the depictions of humans
that are most revealing.
392
00:36:53,817 --> 00:36:58,655
It looks these ones, a load of people here
holding hands, some sort of ceremony maybe.
393
00:37:03,927 --> 00:37:06,463
And here we've got
further evidence of human activity.
394
00:37:07,163 --> 00:37:08,031
Hand prints.
395
00:37:08,998 --> 00:37:13,403
Someone has been here thousands of years ago,
put their hand on this rock here.
396
00:37:15,205 --> 00:37:17,207
And this person's roughly my size.
397
00:37:17,574 --> 00:37:19,375
Look over here, children.
398
00:37:20,710 --> 00:37:22,312
The hands are all intermediate sizes,
399
00:37:22,612 --> 00:37:26,449
suggesting there's a whole community here
living in this area that's now a desert.
400
00:37:27,050 --> 00:37:28,184
Maybe they just wanted to say,
401
00:37:29,285 --> 00:37:32,522
'I was here,' and leave their signature on the rocks.
402
00:37:35,458 --> 00:37:38,728
Some of the human activity scenes
suggest the presence of water.
403
00:37:39,395 --> 00:37:41,331
And this is a lovely figure here.
404
00:37:41,598 --> 00:37:43,867
Some people believe swimming.
405
00:37:44,601 --> 00:37:46,503
And there's a line of these coming along here.
406
00:37:46,736 --> 00:37:49,172
There's another excellent example there.
407
00:38:02,519 --> 00:38:03,753
So what we've got here...
408
00:38:04,053 --> 00:38:10,126
...is savannah animals, human activity,
some of which suggests swimming.
409
00:38:10,360 --> 00:38:13,296
Evidence of communities
and over long periods of time,
410
00:38:13,797 --> 00:38:17,567
which suggests a wet environment
when they were living here in the past.
411
00:38:19,335 --> 00:38:24,674
So where did the waters that sustained
those people and animals come from'?
412
00:38:25,508 --> 00:38:28,311
A day's travel away is the valley of Wadi Bakt.
413
00:38:32,115 --> 00:38:36,319
Here there are clues that
may help to resolve this great mystery.
414
00:38:39,088 --> 00:38:43,827
Layers of sediment are a perfectly preserved
historical record of the climate here.
415
00:38:44,260 --> 00:38:46,529
And what they reveal is astonishing.
416
00:38:47,163 --> 00:38:49,666
Okay, what we got here
is layers of different sediment.
417
00:38:49,899 --> 00:38:51,601
At the bottom here we've got sand.
418
00:38:51,801 --> 00:38:54,504
This sand is much the same
as the sand we see today.
419
00:38:54,737 --> 00:38:59,242
It indicates a dry environment, sand blowing around
and being deposited on the surface.
420
00:38:59,676 --> 00:39:05,048
And then above this we've got this darker,
sort of grey layer of clay.
421
00:39:05,348 --> 00:39:07,083
The clay's been deposited by a river.
422
00:39:07,350 --> 00:39:11,354
There's a river flowing down this valley here,
depositing these sediments.
423
00:39:14,424 --> 00:39:16,192
Well, this is evidence of the monsoon.
424
00:39:16,459 --> 00:39:20,864
We've got seasonally wet, seasonally dry,
seasonally wet, seasonally dry.
425
00:39:21,030 --> 00:39:24,267
Some of them for longer periods of time
where we get a really enhanced monsoon,
426
00:39:24,534 --> 00:39:26,536
some shorter periods of time.
427
00:39:26,870 --> 00:39:28,872
We may just get an annual wetting and drying.
428
00:39:29,839 --> 00:39:30,840
And we know when that was.
429
00:39:31,107 --> 00:39:33,343
These sediments have been dated
through radio carbon dating,
430
00:39:33,543 --> 00:39:37,013
and the sediments right down the bottom there
date to about 10,000 years ago.
431
00:39:37,213 --> 00:39:40,884
And the sediments up the top here
date to 5,500 years ago.
432
00:39:42,218 --> 00:39:47,590
This is 4,500 years of monsoon here in the Sahara,
one of the driest places on Earth today.
433
00:39:50,827 --> 00:39:56,099
Regular monsoon rains fed
a lake in Wadi Bakt for 4,500 years,
434
00:39:56,432 --> 00:39:59,102
so human swimmers are not so unlikely.
435
00:40:01,704 --> 00:40:04,207
Well, what we've got here
is a spectacular looking lake.
436
00:40:05,275 --> 00:40:08,244
We've got these lake sediments stretching
across much of this valley,
437
00:40:08,544 --> 00:40:09,545
from where we can see.
438
00:40:13,216 --> 00:40:13,683
We can see them...
439
00:40:13,883 --> 00:40:17,086
...extending over here to the other
side of the valley and the face of that escarpment.
440
00:40:17,287 --> 00:40:19,322
So we've got a pretty big lake here.
441
00:40:20,790 --> 00:40:24,294
We'd have had grasslands with scattered trees.
442
00:40:24,527 --> 00:40:26,629
Around the edge of these lakes
we'd have had marshes,
443
00:40:26,930 --> 00:40:29,465
we'd have had hippopotamuses and crocodiles
in those lakes.
444
00:40:29,699 --> 00:40:33,870
We'd have had herds of antelopes and wildebeests
roaming across the plains.
445
00:40:34,437 --> 00:40:38,508
Just your average savannah,
but stretching right the way across the Sahara.
446
00:40:41,978 --> 00:40:45,581
Nick's research has revealed that
the ancient African monsoon...
447
00:40:45,848 --> 00:40:51,087
...created an entire network of rivers and lakes
that crisscrossed the Sahara.
448
00:40:51,621 --> 00:40:54,924
These waterways,
some the size of the Caspian Sea,
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00:40:55,191 --> 00:40:57,727
supported communities of animals and people.
450
00:40:59,062 --> 00:41:02,398
But what was it that brought
the monsoon rains to the Sahara'?
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More importantly, what made them cease?
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00:41:07,704 --> 00:41:09,539
The answer lies with the sun.
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00:41:09,906 --> 00:41:16,512
As with the Indian monsoon today, summer heating
across the Sahara acted as a huge heat engine...
454
00:41:16,980 --> 00:41:22,452
...drawing moisture in from the ocean
and depositing it as rain across the continent.
455
00:41:23,086 --> 00:41:28,391
And just as with the Indian monsoon,
the more heating, the stronger the effect.
456
00:41:33,363 --> 00:41:35,999
So during the time of the green Sahara,
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00:41:36,232 --> 00:41:38,935
temperatures must have been
even hotter than today,
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driving a monsoon system into the desert.
459
00:41:45,308 --> 00:41:50,380
Scientists have linked these higher temperatures
with changes to the Earth's angle of tilt.
460
00:41:50,780 --> 00:41:54,951
Today the angle of tilt is 23.4 degrees,
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but every 41,000 years that angle swings
between 22 and 24 and a half degrees.
462
00:42:07,964 --> 00:42:09,866
Back when the Sahara was green,
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the Earth was close to its maximum tilt
of 24 and a half degrees.
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00:42:16,773 --> 00:42:18,674
It may seem like a tiny shift,
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but combined with small cyclical changes in
the direction of the title and the shape of our orbit,
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the results was the sun shone
more intensely over the northern hemisphere...
467
00:42:30,153 --> 00:42:32,622
...and it powered a Saharan monsoon.
468
00:42:34,157 --> 00:42:39,062
Then, five and a half thousand years ago,
the cycles changed.
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00:42:39,362 --> 00:42:43,933
The Earth's tilt decreased again,
moving closer to what it is now.
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The Saharan monsoon stopped, and very quickly
the water and vegetation started to disappear.
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00:42:51,674 --> 00:42:55,545
The green savannah was transformed,
leaving what we see today:
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one of the world's largest deserts.
473
00:42:59,749 --> 00:43:01,617
Recent research suggests that...
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...the Saharan monsoons may have had
an even greater impact on human civilisation.
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Old lake sediments suggest there was
another wet phase in the Sahara's history,
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120,000 years ago.
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00:43:15,965 --> 00:43:18,768
This particular green Saharan episode
is a really exciting one,
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00:43:18,968 --> 00:43:22,738
because it may well have controlled
the migration of humans out of Africa.
479
00:43:24,707 --> 00:43:29,145
Modern humans evolved in Africa
around 200,000 years ago.
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But how they migrated through
the impassible Saharan desert...
481
00:43:33,416 --> 00:43:36,219
...and out of Africa has long been a mystery.
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00:43:37,253 --> 00:43:41,958
A green Sahara 120,000 years ago
may provide the answer.
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00:43:42,992 --> 00:43:46,662
It is at this time when we find
the first evidence of modern humans...
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...to the north of the Sahara,
along the Mediterranean coast.
485
00:43:50,133 --> 00:43:54,637
Presumably, these humans got across that
green Sahara at that time.
486
00:43:55,071 --> 00:43:58,574
And once they're up across the Sahara in the -
in the - along the Mediterranean coast,
487
00:43:58,875 --> 00:44:02,778
it's easy for them to just migrate out
through the Levant and then into Europe,
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00:44:03,045 --> 00:44:05,047
which they did about 50,000 years ago.
489
00:44:08,484 --> 00:44:12,889
It's remarkable to think that
tiny variations in the Earth's tilt...
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00:44:13,222 --> 00:44:17,460
...may have led to one of the most
fundamental movements in human history:
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00:44:17,727 --> 00:44:19,495
our exodus out of Africa.
492
00:44:20,596 --> 00:44:24,433
And equally remarkable to think that
because of the Earth's changing tilt,
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there will come a time
when the Sahara will be green again.
494
00:44:31,507 --> 00:44:34,310
But not for another 15,000 years.
495
00:44:40,750 --> 00:44:44,654
The seasonal cycles of the years
are like Earth's heartbeat.
496
00:44:45,254 --> 00:44:49,659
They give our planet its personality,
range of climates and habitats.
497
00:44:50,226 --> 00:44:53,763
They even provide
the vast numbers of plant and animal species...
498
00:44:53,963 --> 00:44:58,100
...that the ancients ascribed to
mystical power or divine providence.
499
00:44:59,702 --> 00:45:04,273
It's little wonder then that
they celebrated the significant points in a year.
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00:45:04,774 --> 00:45:06,442
The twice-yearly equinoxes,
501
00:45:06,676 --> 00:45:11,447
when equal periods of day and night
mark the divisions between summer and winter.
502
00:45:12,615 --> 00:45:14,450
And the solstices in winter and summer,
503
00:45:14,784 --> 00:45:17,920
where the days reach
their minimum and maximum durations.
504
00:45:22,225 --> 00:45:26,329
June 21st is the longest day
in the northern hemisphere,
505
00:45:26,729 --> 00:45:27,997
the summer solstice.
506
00:45:33,135 --> 00:45:36,872
Here, at Kom Ombo, near the ancient city of Aswan,
507
00:45:37,173 --> 00:45:43,946
the sun worshipping Egyptians were able to celebrate
their deity's finest hour in this magnificent temple.
508
00:45:47,316 --> 00:45:49,452
And because of its location on the globe,
509
00:45:49,685 --> 00:45:54,357
this shrine to the Sun is one of the most
significant places to witness the solstice.
510
00:45:56,492 --> 00:45:58,861
At noon the sun is directly overhead...
511
00:45:59,128 --> 00:46:03,566
...and shines into this water well
making it a perfect light well.
512
00:46:14,744 --> 00:46:19,982
If we were to trace a line parallel with the equator
from Aswan right around the globe,
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00:46:20,349 --> 00:46:25,721
it would be the furthest point north
the midday sun can ever be directly overhead.
514
00:46:30,059 --> 00:46:31,861
This is the Tropic of Cancer,
515
00:46:32,295 --> 00:46:36,332
and because the Earth is tilted at 23.4 degrees
from the vertical,
516
00:46:36,799 --> 00:46:40,936
the Tropic of Cancer
lies 23.4 degrees above the equator.
517
00:46:45,141 --> 00:46:49,145
And the June solstice defines
another significant line of latitude.
518
00:46:49,512 --> 00:46:56,218
As the northern hemisphere points towards the Sun,
the Arctic experiences 24 hours of sunlight.
519
00:46:57,186 --> 00:47:01,257
On the solstice,
the midnight sun reaches its fullest extent,
520
00:47:01,691 --> 00:47:07,930
a line marked by the Arctic Circle,
which is 23 and a half degrees from the North Pole.
521
00:47:16,238 --> 00:47:22,745
It's shaped our destiny and informed our beliefs
about ourselves and our place in the universe.
522
00:47:26,315 --> 00:47:28,250
It's given us terrifying storms.
523
00:47:28,718 --> 00:47:29,785
Oh my god.
524
00:47:32,855 --> 00:47:35,758
It's showered us with benevolent seasonal rains.
525
00:47:38,627 --> 00:47:42,198
It's driven the astounding diversity of life on Earth.
526
00:47:45,901 --> 00:47:49,305
And even enabled the flourishing of our own species.
527
00:47:52,842 --> 00:47:55,277
What was once thought to be mysterious...
528
00:47:55,511 --> 00:48:01,050
...and beyond our comprehension turns out
to be the simple truth of the Earth's tilt.
529
00:48:04,186 --> 00:48:10,226
Our planet is what it is today
because of the small matter of 23.4 degrees.
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00:48:15,831 --> 00:48:21,370
What it will be in the future will be in large part
due to what that number becomes.
52480
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